The armature current signal of a direct current motor comprises a so-called direct component and an alternating component superimposing the direct component. The alternating component is created when operating the direct current motor as a result of the interaction of the magnet (field), the armature winding, and the commutator of the direct current motor. This becomes apparent in a short-term change of the induced voltage, which results in the fluctuation of the armature current signal. The current peaks contained in the armature signal—hereinafter called current ripples—occur during a rotation of the armature at a frequency corresponding to the number of collector segments.
For example, if the armature comprises ten collector segments, ten corresponding current ripples can be observed in the armature current signal. Counting the current ripples can therefore provide information on the present rotational position of the direct current motor armature and thus also with respect to the element driven by the motor within a predetermined range of motion. For this purpose, an analog current signal is digitized in order to be able to perform the corresponding count. By evaluating the frequency of the determined current ripple, it is possible also to determine the armature speed of the direct current motor.
A method for providing a digital current ripple signal, for example, is described in DE 198 34 108 A1. By applying the method described in this document, the armature signal is sampled during the entire power supply period in a sampling frequency that is considerably greater than the expected maximum current ripple frequency. To prevent the disturbing pulses superimposing the armature current signal from being included in the evaluation of a current ripple count, the analog armature current signal is processed correspondingly prior to being digitized, for example, in that the signal is subjected to a frequency filtering process.
In order to detect a current ripple, the armature current signal is differentiated and subsequently subjected to a differential formation of the minimum and maximum values in a predetermined time interval. From these differential values, the current ripples then are approximately determined by searching the maximum of the differential values or by determining the centers of gravity of the differential values.
A high sampling rate, as provided in the object of this document, is used to sample the analog armature current signal in order to provide the digitized current ripple signal. As a result of the subsequent evaluation of this digitized signal for detecting the actual current ripple, a precise current ripple detection over the entire armature current signal can be performed. However, this method has the disadvantage that a rather powerful computer is required.
Another method for providing a digital current ripple signal is known from the analog armature current signal of a powered direct current motor, in which the analog/digital conversion results from the armature current signal are based on a threshold value. This analog/digital conversion, for example, can be performed in an ASIC, so that merely a computer capacity and performance must be available that is necessary for the evaluation. However, the problem with applying this method is the existence of an adequately precise current ripple determination during the motor's start-up or run-down phase, so that the error rate is much greater during these phases than during the transient operating condition.